Why does the somatosensory homunculus have hands next to face and feet next to Genitals? A hypothesis.
Why does the somatosensory homunculus have hands next to face and feet next to Genitals? A hypothesis.
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为什么体感小人的手靠近脸，脚靠近生殖器？
DOI：
10.1162/089976698300016936
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发表时间：
1998
影响因子：
2.9
通讯作者：
Farah,MJ
中科院分区：
文献类型：
--
作者：
Farah,MJ
身体表面到其皮质表征的躯体位置映射是神经系统中地形图映射的范例。它是皮层地图可塑性生理学研究（Kaas，1991; Ramachandran，Stewart，& Rogers-Ramachandran，1992 a，1992 b）和自组织计算研究（Pearson，Finkel，& Edelman，1987;梅尔泽尼希，1987; Willshaw & von der Malsburg，1976）的典型案例。然而，地图并不完美。有两个主要的不连续性：侏儒的手和脸是彼此相邻的，生殖器和脚也是如此（Penfield & Rasmussen，1950; Ramachandran，Stewart，& Rogers-Ramachandran，1992 b）。为什么会这样呢？我在文献中找不到任何解释躯体感觉皮层组织的这种奇怪特征的方法。在这里，我指出，自我组织的机制，结合胎儿在子宫中的正常位置，将使地图向这种组织倾斜。在自组织系统中，相邻的神经元代表了一致的激活源，而躯体位置图的产生是因为相邻的体表受到一致刺激的可能性比不相邻的体表更大。产前自我组织的作用在佛罗伦萨等人最近的一项实验中得到了证实。（1996年）。他们切断了猴胎儿的正中神经，证明修复后，神经终止的模式严重紊乱。尽管如此，初级躯体感觉皮质的微电极绘图显示了正常、有序的地形。这一发现令人惊讶的含义是，皮层躯体定位并不反映外周输入的组织;也就是说，皮层侏儒不仅仅是通过有序的上行输入而保存的更多外周躯体定位组织的结果。相反，产前感觉经验的统计数据是躯体感觉侏儒发生的主要因素。出生前的感觉经验与出生后的感觉经验明显不同的一个方面是手触摸面部和脚触摸生殖器的频率，这可以解释侏儒中的不连续性。胎儿体位时四肢的屈曲姿势使手靠近面部，脚靠近生殖器，如图1所示。正常胎动
The somatotopic mapping of the body surface onto its cortical representation is a paradigm case of topographic mapping in the nervous system. It has functioned as the model case for physiological studies of plasticity in cortical maps (Kaas, 1991; Ramachandran, Stewart, & Rogers-Ramachandran, 1992a, 1992b) and computational studies of self-organization (Pearson, Finkel, & Edelman, 1987; Merzenich, 1987; Willshaw & von der Malsburg, 1976). However, the map is not perfect. There are two major discontinuities: the hands and face of the homunculus are adjacent to one another, as are the genitals and feet (Penfield & Rasmussen, 1950; Ramachandran, Stewart, & Rogers-Ramachandran, 1992b). Why is this? I have been unable to find any explanations of this curious feature of somatosensory cortical organization in the literature. Here I point out that mechanisms of self-organization, in combination with the normal position of the fetus in the womb, will incline the map toward just this organization. In self-organizing systems, adjacent neurons come to represent coincident sources of activation, and the somatotopic map results from the greater likelihood of coincident stimulation to adjacent body surfaces than nonadjacent. The role of prenatal self-organization was demonstrated in a recent experiment by Florence et al.(1996). They cut the median nerve to the hand in fetal monkeys and demonstrated that after repair, the pattern of nerve termination was grossly disordered. Nevertheless, microelectrode mapping of the primary somatosensory cortex revealed a normal, orderly topography. The surprising implication of this finding is that cortical somatotopy does not reflect the organization of peripheral inputs; that is, the cortical homunculus is not simply the result of more peripheral somatotopic organizations being conserved through orderly ascending inputs. Rather, the statistics of prenatal sensory experience is the predominant factor in the genesis of the somatosensory homunculus. One way in which prenatal sensory experience differs markedly from postnatal experience is in the frequency with which hands touch face and feet touch genitals, and this can explain the discontinuities in the homunculus. The flexed posture of the limbs in the fetal position places hands near face and feet near genitals, as shown in Figure 1. Normal fetal movement